The use of hot wires and hot films as anemometer transducers is well known in the prior art. In such devices, a heated resistance element serves as a sensing element and its geometry is used to define its spatial response to impinging fluid flow. The sensing element has a non-zero temperature coefficient of resistance and is maintained at constant resistance and, thus, temperature. Examples of prior art heat loss anemometer transducers, thermal anemometer sensors, and operating circuits therefor, are shown in U.S. Pat. Nos. 3,363,462, 3,352,154, 3 604,261, 3,677,085, 3,900,819, 4,024,761, 4,206,638, 4,279,147 and 4,549,433.
U.S. Pat. Nos. 3,352,154 and 3,677,085 describe radial transducers with strips or wires placed on a cylinder or round edge of a transducer. Referring to U.S. Pat. No. 3,352,154, narrow direction sensing strips are shown parallel to the axis of a cylinder where the strips are within the stagnation region of the cylinder. Such a mounting location is not at all usable in a transducer subjected to an impinging flow vector which shifts along the axial dimension of the transducer geometry. Elongated thin film strips or wires are useful in radial flow but not in axial flow. U.S. Pat. No. 3,677,085 describes imbedded wires, strips and films which are widely separated in pairs on a body having an aspect ratio, body width to body thickness, of almost 3 to 1. The described tandem flow meter geometry is structured for use in a bounded duct or pipe having constrained radial flow against the transducer and not unbounded free flow. We are taught by the specification that an effort is made to conceal each of the sensing elements from flow over the supporting body by the presence of the body itself in acting as a barrier.
U.S. Pat. No. 3,604,261 describes wide segmented cylindrically supported sensing films in a transducer geometry which is designed for impinging radial flow wherein heat transfer distributional change is relied on for direction sensing with part of the sensing surface falling in the turbulent lee behind the supporting cylinder dynamic separation points.
Prior art thermal anemometer transducers, which use no moving parts, have characteristically had some degree of difficulty in realizing a desired close conformity to the "ideal" cosine characteristic together with a smooth and continuous transition from one direction to the opposite direction of flow. The use of electrical "dither" signals and artificial "lobe switching" from side-to-side has helped to reduce axis crossing irregularities. U.S. Pat. Nos. 4,206,638 and 4,279,147 teach us that further improvement has been brought about by the use of a self-induced turbulent wake as a naturally occurring "aerodynamic dither" signal in the axis crossing regions. U.S. Pat. No. 4,549,433, although not describing a thermal anemometer, teaches us how to mount individual flat plate supported sensing elements in a non-stressed manner.
The present invention provides a significant improvement in the angular response or azimuth response of the thermal anemometer transducer together with improvement in the perceived signal-to-noise ratio or effective signal output with respect to transducer heating power input, together with improved control and readout means therefor.